Process for manufacturing a multi-layer tab tape semiconductor device

ABSTRACT

One or two, or more, additional conductive layers, separated from one another (if two or more) and separated from a patterned (signal) conductive layer are formed in a flexible substrate, for mounting a semiconductor die in a semiconductor device assembly. These additional layers are used as separate planes for carrying power and/or ground from outside the assembly to the die, on a separate plane from signals entering or exiting the die. TAB processes are disclosed for cutting, bending and bonding inner and outer portions of selected signal layer traces to respective inner and outer edge portions of the additional conductive layer(s), including a two-stage process of (1) first cutting, bending and tacking the selected traces to the additional layer(s), and then (2) repositioning a bonding tool and securely bonding the selected traces to the additional layer(s). A tool (die pedestal) for aiding in the assembly process is also disclosed.

This is a continuation application of U.S. patent application Ser. No.08/451,561, filed May 26, 1995, by Brian Lynch and John McCormick, nowabandoned, which is a continuation application of. U.S. patentapplication Ser. No. 08/894,031, filed Jun. 4, 1992, now abandoned.

TECHNICAL FIELD OF THE INVENTION

The invention relates to mounting a semiconductor device, or integratedcircuit (IC) to a lead frame, flexible lead frame, or tape, for finalpackaging.

BACKGROUND OF THE INVENTION

Generally speaking, there are three distinct techniques of packaging asemiconductor device, in any case said package having leads or the likeexiting the package for electrically connecting the packaged die toother components, either by mounting directly to a printed circuit boardor by plugging the packaged device into a socket which, in turn ismounted to a printed circuit board. These are: (1) plastic molding; (2)ceramic packaging; and (3) flat packing.

U.S. Pat. No. 5,051,813 (Schneider, et al.), incorporated by referenceherein, provides an example of a plastic-packaged semiconductor device.Present plastic packaging techniques involve molding a plastic "body"around a semiconductor die. Prior to molding, the die is attached to alead frame having a plurality of leads ultimately exiting the packagefor connecting the semiconductor device to external circuits, such asvia conductors on a printed circuit board. Various forms of plasticpacks are known, including DIP (Dual In-line Package), PQFP (PlasticQuad Flat Pack) and PLCC (plastic leaded chip carrier). The lead frameis formed from a single thin layer (foil) of conductive material, whichis punched out to form individual leads. The inner ends of the leads areusually wire bonded to the active side (components, bond pads) of thedie. When handling the lead frame, prior to encapsulation, it isexceptionally important to avoid damaging the closely-spaced, delicateleads.

U.S. Pat. No. 4,972,253, incorporated by reference herein, provides anexample of multi-layer ceramic packages which are laminated structuresof alternate conducting and non-conducting layers, formed of thickconductive film and nonconductive ceramic, respectively. Generally, theconductive layers carry only one of signals, power or ground. Thisapproach, particularly separating the signal plane (layer) from theground and power planes, has distinct electrical advantages, which arewell known. In this type of package, the conductive layers are screenedor otherwise disposed between the nonconductive layers, and a veryrigid, stable package is formed. For the signal-carrying layers, leadtraces are typically screened onto an underlying ceramic layer. A die iseventually disposed into an opening in the package and connected toinner (exposed) ends of the lead traces. Generally, there is littleproblem in damaging the lead traces, since they are well supported by anunderlying ceramic layer. Generally, vias are formed in the package toconnect power and ground planes to particular leads in the signal plane.

U.S. Pat. No. 4,965,702, incorporated by reference herein, providesanother example of a multi-layer package, using polymeric (e.g.)insulating layers and a copper foil (e.g.) for the conductive layers.Again, an object of such a multi-layer package is to provide for anelectrical multilayer conductive package which partitions (separates)the power supply system of the package from the signal transmissionsystem as much as practical in order to optimize the performance ofboth.

These two multi-layer ceramic and polymer packages are also known as"chip carriers". Both are preferably completely formed prior to mountingthe semiconductor die within an opening in the chip carrier, and in boththe inner leads are well-supported. Hence, both of these chip carriersinherently avoid the problem of lead damage during handling and mountingof the die.

FIGS. 1A and 1B show an example of tape-based flat packing. Asillustrated herein, a semiconductor device assembly 10 includes anupper, segmented plastic film layer 14 (formed of segments 14A, 14B, 14Cand 14D), a lower plastic film layer 16, metallic leads 18 sandwichedbetween the two plastic layers 14 and 16, a metallic (preferably copper)die attach pad 20 supported between the two plastic layers 14 and 16, asemiconductor device 22 mounted on the die attach pad 20 and bond leads24 connecting the semiconductor device 22 to the leads 18. It is alsoknown to employ conductive "bumps" on the inner ends of the leads,rather than bond wires, to connect the leads to the semiconductor die22, in a tape automated bonding (TAB) process. The upper and lowerplastic layers are suitably formed of polyimide, and form a thin,insulating supportive structure for the leads 18. A square, insulatingring ("body frame" or "dam") 26 is disposed atop the leads 18 betweenportions 148 and 14C of the upper plastic film layer, outside the diearea. A layer-like quantity of silicone gel 28 is disposed over the die22 and bond wires 24, and acts as an ionic contamination barrier for thedie and as a stress relief for the leads 24 during assembly of thesemiconductor device assembly, and further prevents an ultimateencapsulation epoxy 30 from contacting the semiconductor die. Evidently,the inner ends of the leads 18 are very fragile, and extreme care mustbe exercised when assembling the die 22 to the leads 18. In thisrespect, tape mounting a semiconductor die requires a similar degree ofextreme care when mounting the die to the fine-pitch conductive leads.

Further examples of mounting semiconductor devices to a tape structureare shown in U.S. Pat. Nos. 4,800,419 and 4,771,330, incorporated byreference herein.

As used herein, the term "semiconductor device" refers to a silicon chipor die containing circuitry and bond sites on one face, and the term"semiconductor device assembly" refers to the semiconductor chip andassociated packaging containing the chip, including external packageleads or pins for connecting the semiconductor device assembly to asocket or a circuit board, and including internal connections (such asbond wires, TAB, or the like) of the chip to inner ends of the leads.

The aforementioned patents relate to semiconductor device assemblieshaving a high lead count, which is "de rigueur" in modern semiconductordevices. The plastic packaging and tape mounting techniques aregenerally indicative of methods of mounting semiconductor devices topreformed lead frames having a plurality of extremely delicateconductors connecting to the die.

As mentioned above, there are generally two techniques for connecting adie to inner ends of lead frame conductors, namely wire bonding andtape-automated bonding (TAB). In TAB, "bumps" typically formed of gold,are located on either the die ("bumped die") or on the inner ends of thelead fingers ("bumped tape"). See, e.g., U.S. Pat. No. 4,842,662, FIGS.5 and 6, respectively.

U.S. Pat. No. 4,842,662, incorporated by reference herein, disclosesbonding integrated circuit components directly to a TAB tape, withoutthe intermediary of a gold bump, by use of a process employingultrasonic energy, pressure, time, heat and relative dimensions of theTAB tape. Generally, the end of a lead is "downset" (urged down) onto adie. (See column 6, lines 5-8) This may be thought of as a "bumpless"TAB process.

While the above-referenced patents teach various techniques of forminglead frames, TAB tapes, and the like, and various techniques forconnecting semiconductor dies to same, these techniques generallyinvolve only one layer, or plane, of patterned metal conductors (leadfingers), which single conductive layer is represents a single planecarrying signals, power and ground to the semiconductor die.

As mentioned hereinabove, it is electrically desirable to providedistinct planes for carrying signal, power and ground from leads (orpins) exiting the package to the die within the package.

U.S. Pat. No. 4,933,741, incorporated by reference herein, discloses amultilayer package for integrated circuits having a ground plane (20)electrically isolated from a plane of conductors (14) by means of aninsulating layer (16) formed of polyimide. The ground plane (20) isconnected to selected conductors (14) by means of vias (18) extendingthrough the insulating layer (16). The remaining (non-groundedconductors) carry signals and power to/from the integrated circuitdevice (11). As pointed out therein, " b!ecause of the small physicalsize of the electrical conductors 14, they represent a significantimpedance to operating potential and current 15 applied to theintegrated circuit 11 causing an undesirable voltage drop along thelength of the conductors 14. Additionally, capacitive coupling betweenthe conductors 14 causes cross talk on the conductors 14 which applysignals to and/or derive signals from the integrated circuit 11.Further, the impedance of the conductors 14 create switching noise whenthe DC operating current 15 applied to the integrated circuit varies."And, as noted therein, "the capacitive cross coupling between theconductors 14 can be reduced by a separate! ground plane 20 which alsosurrounds the integrated circuit 11 and is located adjacent theplurality of conductors." (See, especially, column 2, lines 31-46)

Despite the generally accepted notion that providing a separate groundplane has desirable electrical characteristics, the examples set forthabove are limited to rigid, multi-layer ceramic or polyimide or polymerchip carriers. In both of these multi-layer approaches, it is relativelyfeasible to provide vias between separate metal layers and theintervening insulating layers.

On the other hand, in a tape-mounted, flexible substrate, semiconductordevice assembly, it has generally not been very practical to consider orimplement incorporating a distinct ground plane, since this type of"flexible" packaging does not lend itself readily to such a multi-layerapproach employing vias spanning insulating layers.

For example, commonly-owned, copending U. S. patent application Ser. No.07/829,977 abandoned, entitled RIGID BACKPLANE FOR SEMICONDUCTOR DEVICEASSEMBLY, filed on Jan. 31, 1992, by Michael D. Rostoker, discloses anintegrated circuit device package (semiconductor device assembly) havinga flexible substrate including an upper patterned insulative layer, anda lower patterned conductive layer including a plurality of packageleads (lead fingers). The assembly further includes a rigid orsemi-rigid lower protective layer, formed of ceramic, glass, metal,plastic, and combinations thereof, which provides enhanced protectionfrom mechanical and electrical degradation of the packaged device, andwhich may also serve as a heat sink.

Hence, we see that there are at various desirable and unfulfilledobjectives in the design and implementation of tape-mounted,flexible-substrate semiconductor device assemblies.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an improvedsemiconductor device assembly.

It is a further object of the present invention to provide amulti-layer, relatively-flexible, tape-like substrate for mounting asemiconductor device, said substrate having at least a signal layerdistinct from at least a ground plane.

It is a further object of the present invention to incorporate at leastone additional electrically conductive plane into a semiconductor deviceassembly using tape automated bonding (TAB) assembly techniques.

It is a further object of the present invention to provide a rigidsupportive structure in a TAB package.

It is a further object of the present invention to provide an improvedtechnique for manufacturing a semiconductor device assembly.

It is a further object of the present invention to provide tooling forpracticing the inventive techniques disclosed.

According to the invention, a relatively-flexible, tape-like substratefor mounting a semiconductor device has a patterned, conductive layer offine-pitch leads extending into a central area in which a semiconductordie may be connected to the inner ends of the leads. The substrateincludes an underlying insulating (e.g., plastic film) layer supportingthe leads, with an opening larger than the area defined by the innerends of the leads so that inner end portions of the leads remain exposedpast the opening in the insulating layer for connecting the leads to thesemiconductor device. Preferably, all of the leads are connected to thesemiconductor device.

A second, additional conductive layer underlies the insulating layer andis not patterned to form distinct leads, but rather forms a planarring-like layer, the inner edge of which extends past the opening in theinsulating layer, but is larger than the die. Hence, the substrate canbe viewed as a sandwich of two conductive layers, one of which ispatterned into discrete conductors (traces) and the other of which isnot patterned, and an insulating layer interposed between the twoconductive layers.

According to the invention, a first group (portion) of the total numberof lead traces in the patterned conductive layer are connected to thedie, preferably by TAB bonding or similar process (i.e., rather than bywire bonding). A remaining, selected portion of the lead traces in thepatterned conductive layer are also connected at their inner ends to thedie, and are then: (1) broken off at or just within the edge of theopening in the insulating layer, leaving an inner end portion of theselected lead traces disconnected from the remaining portion of theselected lead traces, one end of the inner end portion bonded to the dieand the other end of the inner end portion being a "free" end, and arethen (2) bent downwards past the insulating layer so that the free endsof the inner end portions of the traces contact an inner edge portion ofthe additional conductive layer extending into the opening of theinsulating layer, and are then (3) bonded at their free ends to theinner edge portion of the ring. In this manner, the additionalconductive layer can act as a ground (or power) plane connected to thedie.

The additional conductive layer also extends under window-like slitsnear the outer edges of the insulating layer, where a similar process ofcutting, bending and bonding outer portions of the selected lead tracesto an outer edge portion of the additional conductive layer isperformed. In this manner, external portions of the selected leadtraces, beyond the outer portions, exit the ultimate semiconductordevice assembly, and can be connected to external ground (or power).

Hence, the additional conductive layer can be used to conduct ground (orpower) from external portions of the selected lead traces to inner endportions of the lead traces, to the die, bypassing on a different planethe remaining intermediate portions of the lead traces which areintended (primarily) to carry signals to and from the die. In thismanner, a distinct ground (or power) plane is established which isisolated from the patterned conductive layer (primarily signal paths),and the beneficial electrical characteristics discussed above accrue toa flexible, tape-mounted semiconductor device assembly.

Further according to the invention, two additional conductive layers areformed, one for ground and one for power. In a manner similar to thatset forth with respect to one additional conductive layer, selectedleads are cut, bent and connected to inner and outer edge portions ofone additional conductive layer, and selected other leads are cut, bentand connected to inner and outer edge portions of the second additionalconductive layer.

Further according to the invention, the selected and other selected leadtraces are cut at an edge of the insulating (plastic) layer between thepatterned conductive layer and the first additional (or simply"additional" if only one) conductive layer by urging downward on theselected and selected other lead traces with a bonding tool.

Further according to the invention, in a first bonding step, a bondingtool is used to cut, bend and partially bond a free end of the selectedand selected other (if applicable) traces to the first and second (ifapplicable) additional conductive layers. In a second bonding step, thebonding tool is repositioned and bonds the already stabilized (tacked tothe additional layer) free end of the lead trace to the additionalconductive layer.

Further according to the invention, various methods of TAB bonding aconductive trace to an additional conductive layer, avoiding the use ofvias, is disclosed.

Further according to the invention, various bonding tools for effectingTAB bonding of lead traces to an additional conductive layer aredisclosed.

Further according to the invention, a tool (die pedestal) for aiding inthe assembly of the die to the tape substrate, and for aiding incutting, bending and bonding the selected and selected other lead tracesto the additional conductive layer(s) is disclosed.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a prior art technique of tape-mounting asemiconductor device to a flexible substrate.

FIG. 1B is a cross-sectional view of the prior art technique of FIG. 1A,taken on a line 1B--1B through FIG. 1A.

FIG. 2A is a cross-sectional view of a prior art technique of forming atwo-metal-layer, TAB-type semiconductor device assembly, using vias toconnect the two metal layers together.

FIG. 2B is a top plan view of a portion of the assembly of FIG. 2A.

FIG. 3A is a perspective view, partially in cross-section, of amulti-layer flexible substrate mounting a semiconductor die, with oneadditional conductive layer, according to the present invention.

FIG. 3B is a top plan view of a semiconductor device assembly formedaccording to the technique of FIG. 3A.

FIG. 3C is a cross-sectional view of the semiconductor device assemblyof FIG. 3B.

FIG. 3D is a perspective view, partially in cross-section, of analternate embodiment of the multi-layer flexible substrate mounting asemiconductor die, with one additional conductive layer, according tothe present invention.

FIG. 4A is a perspective view, partially in cross-section, of amulti-layer flexible substrate mounting a semiconductor die, with twoadditional conductive layers, according to the present invention.

FIG. 5A is a perspective view of a prior art thermo-sonic bondingtechnique.

FIG. 5B is a perspective view of a bonding tool and technique, accordingto the present invention.

FIG. 5C is a perspective view of a bonding technique, according to thepresent invention.

FIG. 5D is a perspective view of a bonding technique, according to thepresent invention.

FIG. 5E is a perspective view of an alternate embodiment of a bondingtool, according to the present invention.

FIG. 5F is a perspective view of an another alternate embodiment of abonding tool, according to the present invention.

FIG. 6A is a perspective view of a tool (die pedestal) employed in thebonding technique of the present invention.

FIG. 6B is a cross-sectional view of the die pedestal of FIG. 6A, inuse, and also shows the two-point bonding technique of the presentinvention.

FIGS. 7A-7D are cross-sectional views of heat sink options for thesemiconductor device assemblies of the present invention (e.g., as shownin FIGS. 3A and 4A).

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B show a prior art technique of tape-mounting asemiconductor device to a flexible substrate. As discussed above, asemiconductor die can be wire bonded, or TAB mounted to the inner endsof the conductive leads (traces).

FIGS. 2A and 2B show another prior art technique 200 of tape-mounting asemiconductor die 202 to a flexible substrate 204. In this example, thesubstrate has first metal layer 206, such as copper foil, patterned withconductive lines (traces) 208. These traces 208 are very fine pitch tomeet the signal input/output (I/O) demands of modern, complexsemiconductor devices. Only four traces 208a, 208b, 208c and 208d areshown (see FIG. 2B), as representative of the hundreds of such tracestypically required. These traces 208 extend from an outer end (right, asviewed in the Figures) where they will connect to external sources andcircuitry, to an inner end (left, as shown) where they are bonded to thedie, and usually each trace (e.g., 208a, 208b, 208c and 208d) carries adistinct I/O signal, or power or ground.

As is known, the first metal layer 206 of traces 208 is suitablysupported by an underlying layer 210 of an insulating material, such aspolyimide film. The insulating layer 210 is provided with a centralopening 212, formed by an inner peripheral edge thereof. The first metallayer traces 208 extend into the opening, a suitable distance forbonding to the die. The first metal layer traces 208 also extend beyondthe outer edge 214 of the insulating layer for connection to externalcircuits and systems.

The substrate 204 is also provided with a second metal layer 220, on anopposite side of the insulating layer 210 from the first metal layer206. The second metal layer 220 is not patterned to form traces, but israther a planar ring-like structure having an inner edge 222 alignedwith the inner edge 212 of the insulating layer 210, and an outer edge224 aligned with the outer edge 214 of the insulating layer.

In this semiconductor device assembly 200, the lower conductive layer220 is intended to be a ground plane. In order to effect this function,vias 230 are formed near the inner edge 222 of the lower conductivelayer 220, which vias 230 extend through the layer 220, through thelayer 210, and through the layer 206, and the vias are usually platedthrough so that individual traces 208 may be electrically connected tothe ground plane 220 near the semiconductor device 202. However, as isreadily apparent from FIG. 2B, in order to accommodate such vias 230,which are on the order of 0,004 inches (100 μm) in diameter, it isnecessary to dedicate two adjacent conductive traces, in this case thetraces 208b and 208c to making the connection to the ground plane.Because of the modern drive to very high lead count packages, theindividual traces 208 are preferably on the order of 0.002 inches (50μm) wide and spaced at a 0.002 inch pitch. And, as is readily seen inFIG. 2B, this means that the two conductive traces 208b and 208c aretied together in the area of the via, and hence cannot carry twoseparate signals. This, of course, wastes valuable traces (leads 208),in an environment where the number of distinct traces carrying distinctsignals is a major design criteria.

In a similar manner, vias 232 are formed near the outer edge 224 of thelower conductive layer 220, which vias 232 extend through the layer 220,through the layer 210, and through the layer 206. Again, it willtypically take at least two adjacent traces (e.g., 208b, 208c) toaccommodate such a via, due to sizing restrictions. Nevertheless, theground plane 220 can be connected to conductive lines 208b and 208c nearthe outer edge 214 of the insulating layer 210. In this manner, usingvias 230 near the inner edge 212 of the insulating layer 210 and usingvias 232 near the outer edge 214 of the insulating layer 210, a groundconnection can be made to an external lead (right hand portion of theleads 208), passed by the via 232 to another plane 220, and brought backup to the original layer 206 near the die 202. Electrical benefits willaccrue by virtue of the largely separate ground plane 220.

FIG. 2B also shows that intermediate portions of the traces 208b and208c are preferably excised (or simply not formed), and theseintermediate portions of the traces 208b and 208c are shown in dashedlines. This ensures that the electrical path from one end of the leads208b and 208c to the other ends thereof is largely in the second, offset(from the first 206) plane 220.

Hence, the technique 200 is illustrative of prior art techniques offorming a two-metal-layer, TAB-type semiconductor device assembly, usingvias to connect the two metal layers together. This is commonly called"two metal layer TAB tape". As discussed above, using vias to connectone plane to another will unavoidably reduce the number of distinctleads available for inputting and outputting signals to/from the die.

A further problem with two metal layer TAB tape is that the bottom layer220 is a thin foil, much like the top layer 206 (but not patterned intotraces). Hence, the bottom layer 220 does not provide significantadditional mechanical support for the substrate 204, nor does the bottomlayer 220 provide much in the way of enhanced thermal performance.Further, even if the intermediate portions (dashed lines, FIG. 2B) ofthe conductors 208b and 208c are removed, there is still a significantportion of these conductors in the signal layer 206, therefore allowingparallel paths for ground. Further each tape (substrate) design isspecific to a particular application, i.e., must be customized for theparticular "pin-outs" of a particular semiconduotor device. (The term"pin-outs" is used to indicate which bond site on the die is used forsignals, which is used for power and which is used for ground. Pin-outstypically vary widely from device-to-devices.) Further, the manufactureof such two metal layer TAB tapes is relatively expensive (i.e., ascompared with the inventive techniques disclosed below.

MULTI-LAYER FLEXIBLE SUBSTRATE HAVING A SECOND CONDUCTIVE PLANE

According to the present invention a flexible substrate, such as a TABtape, is provided with a first conductive layer having patterned lines(traces) primarily for carrying signals to and from the die, and isprovided with a second conductive layer for providing power and/orground connections on a plane offset from and insulated from the firstconductive layer. Inasmuch as the first conductive layer carries all ofthe signals (versus power and ground), it is sometimes referred toherein as a "signal layer". The layer 206 of FIGS. 2A and 2B is such asignal layer. The flexible TAB tape substrate, a semiconductor deviceassembly employing same, tools for manufacturing same, and methods ofmanufacturing same are disclosed herein.

FIG. 3A shows a relevant portion of a semiconductor device assembly 300,partially assembled (not encapsulated or lidded), employing theinventive technique of breaking, bending and bonding selected leads to asecond conductive layer using a TAB process.

A first conductive layer 310 is patterned to have a plurality offine-pitch conductive leads (traces) 312, 314 and 316 (only three ofwhat may be hundreds of these leads are shown, for illustrativeclarity). The first conductive layer 310 is supported by an underlyinginsulating layer 320. The first conductive layer 310 may be formed of athin copper foil, on the order of one mil thick. The insulative layer320 may be formed of a thin plastic layer, such as polyimide, on theorder of five mils thick. This is standard for TAB tape semiconductordevice assembly fabrication.

The polyimide layer 320 has a central opening formed by its innerperipheral edge 322, and the opening is sufficiently large toaccommodate a semiconductor die 330. The opening 322 is on the order of1.0 mm (one millimeter) larger than the die 330. This is fairly standardfor TAB tape.

Inner ends 312a, 314a and 316a of the leads 312, 314 and 316,respectively, are each TAB bonded with bumps 318, preferably gold bumps,to "bond sites" 332 on the top circuit-containing surface (circuits notshown) of the die 330. This is a well known procedure. The bumps may beon the leads, on the die, or the inner ends of the leads may be TABbonded to the die without using bumps (See U.S. Pat. No. 4,842,662).

Inner end portions 312b, 314b and 316b of the leads 312, 314 and 316,respectively, span the gap between the outer edge of the die 330 and theinner edge 322 of the polyimide layer 320. As indicated hereinbefore,the distance between the die and the inner edge 322 of the polyimidelayer 320 is on the order 1.0 mm. Generally, this is according toestablished TAB procedures.

As will be seen, the inner end portion of selected leads, in thisexample the lead 312 are broken and bent downward past the polyimidelayer 320 to contact a second conductive plane. This is a markeddeparture from established TAB procedures.

Intermediate portions 312c, 314c and 316c or the leads 312, 314 and 316,respectively, are supported by the polyimide layer 320. This isaccording to established TAB procedures.

According to the invention, a second conductive layer 340 is affixed tothe underside of the polyimide layer 320 (opposite the first conductivelayer 310) using a suitable adhesive 350, such as a 2 mil thick layer ofepoxy. The second conductive layer 340 is formed as a square, relatively(vis-a-vis the layer 310) rigid ring, having an inner peripheral edge342 forming an opening slightly (on the order of 0.5 mm) smaller thanthe opening formed by the inner edge 322 in the polyimide layer 320, andslightly (on the order of 0.5 mm) larger than the die 330. Hence, aninner edge portion 344 of the second conductive layer 340 is exposedwithin the opening formed by the inner edge 322 in the polyimide layer320, on the order of 0.5 mm all around the inner edge 322 of thepolyimide layer 320.

According to the invention, the inner end portion 312b of the selectedleads 312 is severed (cut) at the inner edge 322 of the polyimide layer320. Hence, the inner end portion 312b has one end 312a bonded to thedie 330, and another "free" end 312d. The free end 312d is bentdownward, past the polyimide layer 320, and using a TAB bondingtechnique, either with bumps or without (shown without as is disclosedin the aforementioned U.S. Pat. No. 4,842,662), or other suitable TABbonding technique, the free end 312d of the selected lead 312 is bondedto the exposed inner edge portion 344 of the second conductive layer340. In this manner, a connection is made from selected bond sites 332on the die, via the very short inner end portions 312b of selectedtraces 312, to a second conductive layer 340 which is discrete andoffset from the first conductive (signal) layer 310.

In a similar manner, the selected leads 312 are connected to an outeredge portion 346 of the second conductive layer 340, as follows. Thepolyimide layer 320 has an outer edge 324. The leads 312, 314 and 316extend beyond this edge 326 a suitable distance for allowing connectionof the outer ends 312e, 314e and 316e of the leads 312, 314 and 316,respectively, to external systems and components, such as on a printedcircuit board, or via the intermediary of a socket.

Slightly, on the order of 1.0 mm within the outer edge 324 (i.e., withinthe four outer edges) of the polyimide layer 320, there is provided anelongated slit 326 through the polyimide layer, paralleling therespective edge 324. The slit 326 has an outer edge 326a and an inneredge 326b, and is on the order of 0.5 mm wide (width determined fromouter edge 326a to inner edge 326b).

The second conductive layer 340 extends outward beyond and underneaththe slit 326, so that the outer edge portion 346 of the secondconductive layer 340 is exposed within a "window" formed by the slit326.

Outer end portions 312f, 314f and 316f of the leads 312, 314 and 316,respectively, span the slit 326. The outer end portion 312f of selectedleads 312 (one shown) are severed (cut) at the inner edge 326b of theslit 326. Hence, the outer end portion 312f has one end 312e extendingbeyond the outer edge 324 of the polyimide layer 320, and partiallysupported thereby (by the portion of the polyimide layer between theslit and the outer edge of the polyimide layer), and another "free" end312g. The free end 312g is bent downward, through the slit 326, past thepolyimide layer 320, and using a TAB bonding technique such as isdisclosed in the aforementioned U.S. Pat. No. 4,842.662, or othersuitable TAB bonding technique, the free end 312g is bonded to theexposed outer edge portion 346 of the second conductive layer 340, in amanner similar to the bonding of the free inner end 312d to the exposedinner edge portion 344 of the second conductive layer 340. In thismanner, a connection is made from selected outer lead ends 312e whichare external to the semiconductor device assembly, via relatively shortouter end portions 312f, to the second conductive layer 340.

FIGS. 3B and 3C show a more complete view of the entire semiconductordevice assembly 300 than is shown in FIG. 3A. However, only three leads312, 314 and 316 are illustrated, for clarity of Illustration.

In FIG. 3B, we see more clearly that the polyimide layer 320 is formedas a square ring. As mentioned hereinbefore, the polyimide layer 320 isprovided with a central opening formed by its inner peripheral edge 322,which opening is on the order of 1.0 mm larger than the die 330, allaround the outer periphery of the die 330. And, as mentionedhereinbefore, the inner edge portion 344 of the second conductive layer340, which is also formed as a square ring, extends from under thepolyimide layer 320 to about midway between the inner edge 322 of thepolyimide layer and the outer periphery of the die 330.

The "second conductive plane" established by the second conductive layer340 is suitably employed to providing either ground or power fromexternal supplies to the die, while the first conductive layer havinglead traces is employed more exclusively (than with TAB tapes havingonly one conductive plane) for signals entering and exiting thesemiconductor device from external sources. Preferably, the secondconductive plane is connected to ground, for the electrical benefitsthat will accrue, as discussed hereinabove.

Evidently, if the second conductive layer is used for groundconnections, it can be used for all of the ground connections to thedie. This may amount to about 25 leads (e.g., 312) out of about 400total leads (312 plus 314 plus 316) on the TAB tape substrate. And,another about 25 of the remaining (non-ground) leads in the signal layer(310) may be suitably employed for power, leaving the majority of theleads available for signal I/O.

Hence, as can be seen from FIGS. 3A-3C, a separate, second conductiveplane is established for ground (or power) connections to asemiconductor device, and the second conductive plane is offset andinsulated from a first conductive plane having signal traces.Significantly (considering an area defined by the polyimide, within theslits 326), there are no traces (e.g., 312) or portions thereof (namely312c) providing a ground (or power) connection for the semiconductordevice. In other words, in the first conductive plane (e.g., 310) theconductive traces carry only signals (and power) to and from thesemiconductor device, and none of the intermediate portions (312c) ofthe traces are connected to ground (or power)--i.e., there are no"parallel" paths in the signal layer as there are in the two metal layerTAB tape shown in FIGS. 2A and 2B.

By way of definition, the term "substrate" is used to refer to theassembled layers 310, 320 and 340. The term "partial substrate" is usedto refer to only some of those layers. Preferably, the epoxy layer 350does not extend completely to the edges 322 and 326b of the polyimidelayer, but is spaced about 1.0 mm inward of those edges.

In FIG. 3A, it is evident that the inner and outer ends of theintermediate portions 312c may become "stretched" over the respectivepolyimide edges 322 and 326b when the respective inner and outer endportions 312b and 312f of the traces 312 are cut and bent downward. Thisis acceptable, but is not necessary.

It is evident from FIG. 3A, that the inner end portions 312b areconnected to the inner edge portion 344 of the second conductive layerin an "opposite" orientation than the outer end portions 312f areconnected to the outer edge portion 346. However, in both cases, thefree ends 312d and 312g are oriented towards each other and towards thedisconnected intermediate portions 312c underneath which the secondconductive layer 340 is disposed.

Although discussed in greater detail hereinbelow, it is important tonote that the present technique differs from that disclosed in theaforementioned U.S. Pat. No. 4,842,662 in at least one very significantway. Generally, in the aforementioned patent, an already free end of alead (24) is simply "downset" and bonded to a die (10). In the presentinvention, free ends (312d and 312g) are created by the action offorcing the lead down past the respective edge (322 and 326b) of thepolyimide layer 320. In other words, the selected leads 312 are cutalong their length in order to create the free ends 312d and 312g. Thesefree ends 312d and 312g are not the "normal" free end 312a that theaforementioned patent is intending to bond to a die.

The preferred sequence of assembly is:

(a) provide a tape (partial substrate) having only signal traces (e.g.,the first conductive layer 310) and the plastic support layer (e.g., thepolyimide layer 320);

(b) locate the die (330) in the central opening (formed by edges 322) inthe plastic support layer;

(c) connect the die to the inner ends of the signal traces (e.g., 312and 314) using a TAB process (either bumped or bumpless);

(d) affix a second conductive plane (e.g., 340), having a smallercentral opening (formed by inner edge 342) than the central opening inthe plastic support layer, using a suitable adhesive (350), to theplastic support layer opposite the first conductive layer, so that aninner edge portion (344) of the second conductive layer is exposed inthe opening of the plastic support layer;

(e) break, bend and bond free ends (312d) of inner end portions (312b )of selected traces (312) to the exposed inner edge portion (344) of thesecond conductive layer;

(f) break, bend and bond free ends (312g) of outer end portions (312f)of the same selected traces to an outer edge portion (346) of the secondconductive layer exposed by a slit (326) near the outer edge of theplastic support-layer; and

(g) complete assembly of the semiconductor device assembly using normalTAB process flow, i.e., encapsulating the die, etc.

The steps (e) and (f) are preferably performed with a thermosonic TABbonding process, discussed hereinbelow. However, such bonding may bebumpless, or may be reflow or thermocompression bonding, or may employbumps. Reflow bonding usually involves tin-on-tape, and gold bumps.Thermocompression bonding usually employs gold bumps, combined withforce and high temperature.

The advantages of the present inventive technique over the two metallayer TAB tape (FIGS. 2A and 2B) include reduced cost, designflexibility, added mechanical support for the substrate and the finishedsemiconductor device assembly, no limitations on inner lead via holepitches (compare FIG. 2B), better electrical performance by avoidingparallel ground paths, and better thermal performance.

Regarding design flexibility, it is evident that a "generic" TAB tapecan be formed for a variety of semiconductor devices with various"pin-outs" (which pins are designated for signal, power and ground), andthen (later) certain leads (312) can be selected for connection to thesecond (ground) plane. This is not possible with the two-metal-layer TABtape of FIGS. 2A and 2B, which must be customized for each semiconductordevice having a different pin-out.

Further, with the addition of a second electrically unique plane into aTAB package, the semiconductor device assembly will perform better, theratio of input/output connections versus ground connections will bereduced, and mechanical support will be added to a relatively flexiblepackage. Obtaining these advantages at the relatively low cost affordedby the present inventive technique makes for an attractive semiconductorpackaging technique.

The second conductive layer 340 can be made of any electricallyconductive material. The thickness of the second conductive layer canrange from very thin, on the order of one mil, to very thick, on theorder of one inch. In any case, it is evident that the thickness of thesecond conductive layer 340 can be established as thick as desired, toprovide additional support for the substrate and to provide enhancedthermal characteristics for the substrate, much more so than the secondconductive foil layer employed in the two-metal-layer TAB tape shown inFIGS. 2A and 2B.

FIG. 3D shows an alternate embodiment of a substrate 370, using the samecomponents as the substrate 300 of FIG. 3A, but with the secondconductive layer on top rather than below the first conductive layer. Inthis embodiment, the plastic layer 320 is disposed atop (on the otherside of) the lead layer 310, and the second conductive layer 340 isdisposed atop (rather than underneath) the plastic layer 320.

MULTI-LAYER FLEXIBLE SUBSTRATE HAVING SECOND AND THIRD CONDUCTIVE PLANES

FIGS. 2A and 2B illustrated a prior art technique of "two-metal-layerTAB tape", wherein an additional (second) foil layer was added andconnected by vias through an insulating layer to the first, patterned,conductor layer. The disadvantages and limitations of such a techniquehave been discussed.

It is also known to provide a "three-metal-layer TAB tape" by adding yetanother foil layer to the two-metal-layer TAB tape. Evidently, this willcreate the need for yet more vias, thereby even more significantlyreducing the number of lead traces available for I/O, and will sufferfrom the same limitations and disadvantages, discussed above, that applyto two-metal-layer TAB tape.

According to the present invention, a second and a third conductivelayer are added to a TAB substrate, for carrying ground and power, sothat these currents are isolated from the first signal layer. Thedisclosed technique is similar in many regards to that disclosed withrespect to FIGS. 3A-3C.

FIG. 4A shows relevant portion of a semiconductor device assembly 400,partially assembled (not encapsulated or lidded), employing theinventive technique of breaking, bending and bonding selected leads to asecond and a third conductive layer using a TAB process. It will beappreciated, from the description that follows, that selected leads maybe bent and bonded to one additional conductive layer, and may beconnected by vias to another conductive layer. However, using vias iscontrary to the general purpose of the present invention, which is toefficiently utilize the conductive traces, one by one, rather than twoby two.

A first conductive layer 410 is patterned to have a plurality offine-pitch conductive leads (traces) 412, 414 and 416 (only three shown,for illustrative clarity). The first conductive layer 410 is supportedby an underlying insulating layer 420. The first conductive layer 410may be formed of a thin copper foil, on the order of one mil thick. Theinsulative layer 420 may be formed of a thin plastic layer, such aspolyimide, on the order of five mils thick.

The polyimide layer 420 has a central opening formed by its innerperipheral edge 422, and the opening is sufficiently large toaccommodate a semiconductor die 430. The opening 422 is on the order of2.0 mm (one millimeter) larger than the die 430. Generally, the openingformed by the inner edge 422 of the polyimide layer must be twice aslarge as the corresponding opening 322 of the FIG. 3A embodiment.

Inner ends 412a, 414a and 416a of the leads 412, 414 and 416,respectively, are each to "bond sites" 432 on the top of the die 430,preferably using bumped or bumpless TAB techniques.

Inner end portions 412b, 414b and 416b of the leads 412, 414 and 416,respectively, span the gap between the outer edge of the die 430 and theinner edge 422 of the polyimide layer 420.

As will be seen, the inner end portions of selected leads, in thisexample the leads 414 and 412 are broken and bent downward past thepolyimide layer 420 to contact second and third conductive planes,respectively.

Intermediate portions 412c, 414c and 416c or the leads 412, 414 and 416,respectively, are supported by the polyimide layer 420.

According to the invention, a second conductive layer 440 is affixed tothe underside of the polyimide layer 420 (opposite the first conductivelayer 410) using a suitable adhesive 450, such as a 2 mil thick layer ofepoxy. The second conductive layer 440 is formed as a square, relatively(vis-a-vis the layer 410) rigid ring, having an inner peripheral edge442 forming an opening slightly (on the order of 0.5 mm) smaller thanthe opening formed by the inner edge 422 in the polyimide layer 420.Hence, an inner edge portion 444 of the second conductive layer 440 isexposed within the opening formed by the inner edge 422 in the polyimidelayer 420.

According to the invention, the inner end portion 414b of the selectedlead 414 is severed (cut) at the inner edge 422 of the polyimide layer420. Hence, the inner end portion 414b has one end 414a bonded to thedie 430, and another "free" end 414d. The free end 414d is bentdownward, past the polyimide layer 420 and, preferably using a bumplessTAB bonding technique, the free end 414d of the selected lead 414 isbonded to the exposed inner edge portion 444 of the second conductivelayer 440. In this manner, a connection is made from selected bond sites432 on the die, via the very short inner end portions 414b of selectedtraces 414, to a second conductive layer 440 which is discrete andoffset from the first conductive (signal) layer 410.

So far, this is all very similar to the second conductive plane 340 ofFIG. 3A.

In this example 400, a third conductive plane 460 is provided beneaththe second conductive plane 440, and is electrically isolated therefromby an insulating layer 470, as shown. The insulating layer is suitably apolyimide layer, but can be an adhesive (e.g., epoxy). In practice, asub-assembly comprising the second conductive layer 440, the insulatinglayer 470 and the third conductive layer 460 may be formed separatelyfrom the remainder of the substrate (i.e., the layers 410 and 420), andthen bonded thereto with the adhesive 450. Ultimately, as will becomeevident, the second conductive layer can be used for providing powerfrom an external source to the die, and the third conductive layer 460can be used for providing a ground connection to the die. In thismanner, the layers 440 and 460, separated by the insulating layer 470form a built-in (within the ultimate packaged semiconductor deviceassembly) capacitor for the power and ground connections, which hasnumerous electrical advantages. To this end, the layer 470 can beselected from suitable materials of suitable dielectric constant andthickness to establish a desired built-in capacitance.

The third conductive layer 460 is preferably formed of metal, and may bethicker than a foil in a manner similar to that concerning the secondconductive layer 440. The third conductive layer 460 has a centralopening defined by its inner edge 462 which is on the order of 0.5 mmsmaller than the opening formed by the edge of the second conductivelayer 440. In this manner, an inner edge portion 464 of the thirdconductive layer 460 is exposed within the openings in both thepolyimide layer 420 and the second conductive layer 440.

According to the invention, the inner end portion 412b of a selectedlead 412 is severed (cut) at the inner edge 422 of the polyimide layer420. Hence, the inner end portion 412b has one end 412a bonded to thedie 430, and another "free" end 412d. The free end 412d is bentdownward, past the polyimide layer 420 and, preferably using a bumplessTAB bonding technique, the free end 412d of the selected lead 412 isbonded to the exposed inner edge portion 464 of the third conductivelayer 460. In this manner, a connection is made from selected bond sites432 on the die, via the very short inner end portions 412b of selectedtraces 412, to a third layer 460 which is discrete and offset from boththe first conductive (signal) layer 410 and the second conductive layer440.

Evidently, to implement this breaking and bending of leads (412 and 414)to two additional levels (460 and 440, respectively), it is importantthat the inner end portions of the leads being bent downward to thethird conductive layer 460 be sufficiently long to reach same.Particular dimensions will depend upon particular applications,especially upon the thickness of the second conductive layer 440.

In the manner set forth above, it is taught how selected leads connectedto the die can be cut, bent and connected to two additional conductiveplanes (layers), especially for making power and ground connections. Ina manner similar to that shown in FIG. 3A, the outer ends of theselected leads are also connected to the outer edge portions of theadditional two conductive planes.

The selected leads 414 are connected to an outer edge portion 446 of thesecond conductive layer 440, as follows. The polyimide layer 420 has anouter edge 424. The leads 412, 414 and 416 extend beyond this edge 426 asuitable distance for allowing connection of the outer ends 412e, 414eand 416e of the leads 412, 414 and 416, respectively, to externalsystems and components, such as on a printed circuit board, or via theintermediary of a socket.

Slightly, on the order of 1.0 mm within the outer edge 424 (i.e., withinthe four outer edges) of the polyimide layer 420, there is provided anelongated slit 426 through the polyimide layer, paralleling therespective edge 424. The slit 426 has an outer edge 426a and an inneredge 426b, and is on the order of 1.0 mm wide (twice the width of theslit 326, FIG. 3A).

The second conductive layer 440 extends outward partially, such as atleast 0.5 mm, underneath the slit 426, so that the outer edge portion446 of the second conductive layer 440 is exposed within a "window"formed by the slit 426. Outer end portions 412f, 414f and 416f of theleads 412, 414 and 416, respectively, span the slit 426.

The outer end portion 414f of selected leads 414 (one shown) are severed(cut) at the inner edge 426b of the slit 426. Hence, the outer endportion 414f has one end 414e extending beyond the outer edge 424 of thepolyimide layer 420, and partially supported thereby (by the portion ofthe polyimide layer between the slit and the outer edge of the polyimidelayer), and another "free" end 414g. The free end 412g is bent downward,through the slit 426, past the polyimide layer 420, and is bonded to theinner exposed (through the slit 426) edge 446 of the second couductivelayer 440. In this manner, a connection is made from selected outer leadends 414e which are external to the semiconductor device assembly, viarelatively short outer end portions 414f, to the second conductive layer440.

In contrast to the structure of FIG. 3A, wherein the second conductivelayer 340 extended fully past the slit 326, in this embodiment 400, thesecond conductive layer 440 extends only partially (e.g., halfway) intothe slit area. As we will see, the remaining half of the slit area isrequired for connecting to an exposed (through the slit 426) outer edgeportion 466 of the third conductive layer 460.

The outer end portion 412f of selected leads 412 (one shown) are severed(cut) at the inner edge 426b of the slit 426. Hence, the outer endportion 412f has one end 412e extending beyond the outer edge 424 of thepolyimide layer 420, and partially supported thereby (by the portion ofthe polyimide layer between the slit and the outer edge of the polyimidelayer), and another "free" end 412g. The free end 412g is bent downward,through the slit 426, past the polyimide layer 420, past the secondconductive layer 440, and is bonded to the inner exposed (through theslit 426) edge portion 466 of the third conductive layer 460. In thismanner, a connection is made from selected outer lead ends 412e whichare external to the semiconductor device assembly, via relatively shortouter end portions 412f, to the third conductive layer 460.

Whereas in FIG. 3A we saw that the second conductive layer 340 extendedto the outer edge of the polyimide layer 320, to the edge 324 thereof,we have seen that in this case 400 such is not feasible since it isdesired to leave access past the second conductive layer 440 to thethird conductive layer 460. Hence, as illustrated in FIG. 4A, it isshown to dispose a suitable spacer block between the top surface of thethird conductive layer 460 and the lower surface of the polyimide layer420, in an unsupported area of the polyimide layer 420 between its outeredge 424 and the edge 426a of the slit 426. The spacer block 480 can beformed as a separate element, and bonded with the epoxy 450 to theunderside of the polyimide layer 420, or it can be an integrally formedpart of the third conductive layer 460.

In the example shown in FIG. 4A, the ends of the intermediate portionsof the cut and bent conductors 412 and 414 are shown not stretched overthe respective edges 422 and 426b of the polyimide layer, for clarity.As stated hereinbefore, it is not necessary that these ends be stretchedand bent over.

With reference to FIGS. 3A and 30, which showed that the secondconductive layer 340 could be disposed atop the signal layer 310, it isalso possible to dispose one or both of the second and third conductivelayers 340 and 360 atop, rather than underneath, the signal layer 410.For example, the third conductive layer 460 could be disposed atop thesignal layer 410, while the second conductive layer 440 remains disposedunderneath.

The preferred sequence of assembling the TAB tape having two additionallayers (440 and 460) is:

(a) provide a tape (partial substrate) having only signal traces (e.g. ,the first conductive layer 410) and the plastic support layer (e.g., thepolyimide layer 420);

(b) locate the die (330) in the central opening (formed by edges 422) inthe plastic support layer;

(c) connect the die to the inner ends of the signal traces (e.g. , 412and 414);

(d) sub-assembly the second and third conductive layers (440 and 460)together, including an insulating layer (470) therebetween, and a spacerelement (480) if required;

(e) affix the sub-assembly (440, 470, 460, 480) to the plastic supportlayer opposite the first conductive layer, using a suitable adhesive(450);

(f) break, bend and bond free ends (414d) of inner end portions (414b)of selected traces (414).to the exposed inner edge portion (444) of thesecond conductive layer (440), and break, bend and bond free ends (412d)of inner end portions (412b) of selected other traces (412) to theexposed inner edge portion (464) of the third conductive layer (460);

(g) break, bend and bond free ends (414g and 412g) of outer end portionsof the same selected and selected other traces (414 and 412) to outeredge portions (446 and 466) of the second and third conductive layers(440 and 460), through a slit (426) near the outer edge of the plasticsupport layer; and

(h) complete assembly of the semiconductor device assembly using normalTAB process flow, i.e., encapsulating the die, etc.

The steps (f) and (g) are preferably performed with a thermosonic TABbonding process, discussed hereinbelow. However, such bonding may bebumpless or may employ bumps.

In any of the embodiments described above, it is clearly possible thatthe conductors are bonded to the die and to the additional (second andthird) conductive layers using bumps, solder balls, or the like, ratherthan a bumpless TAB process. Nevertheless, the inventive concept ofcutting, bending and bonding to underlying additional conductive layersis entirely applicable to non-TAB flexible substrates, however they maybe formed.

THERMOSONIC BONDING PROCESS FOR FABRICATING MULTI-LAYER FLEXIBLESUBSTRATES HAVING SECOND AND THIRD CONDUCTIVE PLANES

As mentioned previously, U.S. Pat. No. 4,842,662 is primarily directedto a "downset" operation whereby an already free end of a conductor isbumpless-bonded to a die.

As further mentioned previously, such a technique is suitable forbonding the free ends 312a, 314a and 316a of the conductors 312, 314 and316, respectively (FIG. 3A) and the free ends 412a, 414a and 416a of theconductors 412, 414 and 416, respectively (FIG. 4A) to a die 330.

And, as suggested previously, there is a different and more preferredway to bond the "created" free ends 312d, 312g, 412d, 412g, 414d and414g to second and third conductive layers. As mentioned previously, thepresent method is different from that of the U.S. Pat. No. 4,842,662 inthat the free ends to be bent towards and bonded to the second and thirdconductive layers must first be cut --and this cutting operation makesuse of the sharp edges (322, 326b, 422, 426b) of the polyimide layer(320, 420).

Beyond the cutting operation that creates the free ends to be bonded,the free ends are bonded in a manner different than that of theaforementioned U.S. Pat. No. 4,842,662.

For example, the tool used in the present inventive manufacturing isdifferent than that of the U.S. Pat. No. 4,842,662. A tool is not shownin that patent, but a depiction of a bond formed with the tool is shownin FIG. 8, therein. And the text describes that "the width of the headof the bonding tool should be greater than the width of the TAB tape(conductor) which the head of the bonding tool presses upon." (Column 6,lines 58-61)

By way of further example, with reference to FIG. 5A herein, it wouldappear that the bonding tool 502 implied, but not shown, in the U.S.Pat. No. 4,842,662 has a lead 504 with a widthwise channel 506 extendingtransversely (widthwise) across the head 504. This is evident from thedescription of a single stroke of the bonding tool in the patent, aswell as from the text describing how it is "highly preferred" that "theultrasonic energy is applied along the long axis of the TAB tape beingattached to the pad. When this is done, the resulting relative motioninduced between the tape and the pad produces a rapid longitudinal`wiping` of the two surfaces along this axis." (see column 7, lines13-22).

FIG. 5A, herein, shows such a bonding tool 502 having a width "W" largerthan the width "w" of the conductor 508 being bonded. A two-headed arrow"L" shows the bonding tool vibrating in the longitudinal axis of theconductor 508, and the coaxial motion "1" (lower case `el`) imparted tothe conductor for the stated "wiping". FIG. 8 in the patent shows aconductor after bonding, and the raised ridge left by the transversechannel 506 on the conductor 508 (24 in the patent) is evident. Thischannel 506 would help keep the conductor 508 and the head 504 of thetool 502 moving together as one, longitudinally, to impart the desiredwiping action of the conductor against the bond pad (26 in the patent),rather than allowing the tool to vibrate longitudinally with respect tothe conductor. In other words, the head 504 of the prior art tool isspecifically formed to control longitudinal motion of the conductor.

In contrast to the bonding tool of the prior art, the bonding tool ofthe present invention is specifically formed to control transverse(widthwise) motion of the conductor as it is cut and bent, andultimately bonded to the second or third conductive layer (e.g., 440 or460).

Particularly with respect to the cutting operation, which occurs as thetool head is pressed down against the inner end portion (e.g., 312b ) ofthe conductive lead (e.g., 312) and is sheared off by the edge (e.g.,322) of the polyimide layer (e.g., 320), it is extremely important thatthe conductor does not displace itself widthwise. Hence, the bondingtool of the present invention is designed to prevent transverse(widthwise) movement of a conductive lead (trace) being cut, bent andbonded.

FIG. 5B shows the bonding tool 520 of the present invention. The lead522 of the tool is wedge shaped, having a straight, partially flat edge524 extending widthwise across a conductor 526 (e.g., 312) being cut,bent and bonded. A polyimide layer 528 (e.g., 320) is shown supportingthe conductor along an intermediate portion 526c (e.g., 312c) thereof.The tool 520 is shown coming down on an inner end portion 526b (e.g.,312b,) of the conductor 526, closely adjacent an edge 530 (e.g., 322) ofthe polyimide layer 528. (The numbers in parentheses are crossreferences to exemplary FIG. 3A. The tool 520 and process describedherein can be applied as well to cutting, bending and bonding theselected conductors 412 and 414 in FIG. 4A.)

According to the invention, in order to prevent relative widthwisemotion between the bonding tool head 522 and the conductor 526 beingcut, bent and bonded, the head 524 is provided with a longitudinalgroove 532 extending into the head 522 from the widthwise edge 524thereof. Preferably, as in the prior art (FIG. 5A) the width of thebonding tool head is greater than the width of the conductor beingbonded. In the case of the bonding tool 520, the longitudinal groove 532has a width on the order of 20-33% of the width of the conductor 526, sothat it can easily be located widthwise in the center of the conductorwith allowances for minor misalignments. The vertical depth of thegroove 532 is on the order of 10-20% of the thickness of the conductor526.

FIG. 5C shows the inner end portion 526b of the conductor 526 havingbeen bonded to a an exposed inner edge portion 544 (e.g., 344 of FIG.3A) of a second conductive layer 540 (e.g., 340 of FIG. 3A). The tool520 has been lifted away (not visible). There is, however, visible, araised ridge 560 visible on the center (widthwise) top surface of theinner end portion 526b, running longitudinally (lengthwise) along theconductor 526 and located near the freed (by cutting) end 526d.

It is also possible that a longitudinal groove, such as the groove 532could be added to the bonding tool 502 of the prior art (FIG. 5A), inwhich case there would be a transverse channel 506 as well as alongitudinal groove 532 in the head of the tool. This would preventrelative motion between the tool head and the conductor, in both thetransverse (by the groove 532) and the longitudinal (by the channel 506)directions, and would leave a telltale cruciform-shaped raised ridge onthe conductor. This is shown in FIG. 5E.

More importantly, however, is to take into account the demands made bythe present invention on the tasks accomplished by the bonding tool. Inthe first instance, when the tool head bears down on the conductor(526), its job is to cause the conductor to break, or be cut, as nearlyas possible to the edge (530) of the polyimide layer (528).

According to the invention, in such a "first" stroke of the tool, theconductor (526) is broken and lightly tacked (partially bonded) to theunderlying second (or third) conductive layer (e.g., 340/540 or 560),close to the polyimide edge (530).

Further according to the invention, in order to ensure a good bondbetween the free end (526d) of the conductor and the underlyingadditional conductive layer, the tool is then lifted away from theconductor, repositioned, and brought to bear a second time onto thelightly tacked down conductor.

FIG. 5D shows the results of such a two-stroke bonding process. Asshown, the conductor 526 is, in a first cutting/tacking stroke of thetool (e.g., 520) lightly tacked to the underlying conductive layer 540at a position shown by the dashed line 570 close to the edge 530 of thepolyimide layer helping to cut the conductor. The tool is then liftedand repositioned. Then, in a second bonding stroke, the tool is urgedagainst the already lightly tacked conductor at a position indicated bythe dashed line 572 slightly (e.g., 0.01 mils) further away from thepolyimide edge 530.

The two-stroke bonding process described and shown in FIG. 5D, ensuresthat the bonding tool is pot required to cut, bend and bond, all in onestroke. Nor is the bonding tool required to bend and bond in one stroke,as it is in U.S. Pat. No. 4,842,662. Rather, the bonding tool isrequired only to cut and bend, and lightly tack, in a first stroke,whereupon the conductor being bonded is already stablely (relativelyimmovably) placed in contact with the surface to which it is beingbonded. Then, the bonding tool can perform bonding, without thepossibility of the conductor moving. Taking the stability of the lightlytacked (first stroke) conductor into account, it is possible that thehead of the bonding tool is not provided with any grooves or channels atall, but rather is simply wedge shaped to perform efficient, contiguous(non-interrupted by grooves or channels) bonding.

It is also possible that the bonding tool could simply be a wedge, asshown in FIG. 5F. In this case, the tool head 580 is a simple wedge,with a non-grooved, non-channeled widthwise edge 582 bearing down on theconductor. Inasmuch as the first stroke is not required to effect a goodbond, the simple wedge shape of the tool head 580 will provide moreuniform pressure across the width of the conductor.

Irrespective of the advantages of using a two-stroke, or two-pointcutting/bending/bonding process as described above, it is possible thata single point bonding process, such as is described in U. S. Pat. No.4,842,662, or the like, will suffice. A single or double strokethermo-compression, versus thermo-sonic or fellow, process will alsowork.

DIE PEDESTAL STAGES FOR FABRICATING MULTI-LAYER FLEXIBLE. SUBSTRATESHAVING SECOND AND THIRD CONDUCTIVE PLANES

FIG. 6A shows a pedestal 600 for use in supporting the die and thesubstrate when breaking, bending and bonding the leads. Compare FIG. 3A.The pedestal is essentially a "jig" to aid in bending the conductors,while preventing them from contacting the peripheral edge of the die. Ifthe conductors were to contact the edge of the die, they would becomeshorted thereto. FIG. 6B shows the pedestal 600 in use.

As shown in FIG. 6A, the pedestal 600 has four walls 602, 604, 606 and608 forming a square-tubular open structure. Given that mostsemiconductor dies are square, this is an appropriate shape to surroundthe peripheral edge of a die.

As mentioned hereinabove, the die is typically brought up to the tapesubstrate for bonding the bottom surfaces of the inner ends (e.g., 312a,412a or 414a) of the conductive traces (e.g., 312, 412 or 414) to thetop surface of the die, preferably using a bump-TAB process.

And, as mentioned hereinabove, the inner end portion (e.g., 312b, 412bor 414b) of the conductive trace is urged downward by a bonding tool tocause the inner end portion to sever from an intermediate portion (e.g.,312c, 412c, 414c) which is supported by an insulating layer (e.g., 320,420), so that a free end (e.g., 312d, 412d, 414d) can be bonded to asecond (e.g., 340, 440) or third (e.g., 460) conductive plane.

This is illustrated best in FIG. 6B, which shows a die 630, a conductivetrace 612 bonded at one end 612a to the top of the die with a bump 632.Inasmuch as this aspect of the invention relates to severing/bending/andbonding to an offset second or third conductive layer 640, the polyimidelayer is not shown (refer to FIGS. 3A or 5A).

FIG. 6B shows a bonding tool (e.g., 520) in a first position 620 bearingdown upon the free end 612d of the conductor 612, during a preferredfirst cut/bend/tack stroke, and shows the bonding tool in a secondposition 620' offset from the first position, coming down onto theconductor 612 during a preferred second bonding stroke.

The pedestal 600 is provided with a base 660 at one end of the tubularopening formed by the walls 602, 604, 606 and 608. In FIG. 6B, it isclearly seen that the base 660 can be thinner in a region within thewalls, and thicker in a region without the walls.

The thickness of the base portion 660a within the walls is establishedso that the walls are slightly, such as 0.05 mils higher than thethickness of the die. In this manner, the walls extend slightly abovethe top surface of the die, to prevent the conductor 612 from cominginto contact with the edge of the die, especially when it is being cutand bent (620).

The thickness of the base portion 660b is established to support theadditional conductive layer 640 and may, as stated above, be thickerthan the portion 660a in the event that the bottom surface of the dieextends lower than the bottom surface of the additional conductive layer640. Of course, the opposite could be true, in which case the portion660a may well be thicker than the portion 660b.

The walls 602, 604,606 and 608 are spaced apart from the die a smallamount, on the order of 0.25 mils, to allow the die to be easily placedyet reasonably accurately retained within the opening formed by thewalls. Damaging the die at this stage of the fabrication process, by tootight of a fit, is not a very good idea.

As is best seen in FIG. 6B, the top edges (604a) of the walls arepreferably rounded, as well as extending above the top surface of thedie. This is to ensure that when the conductor is bent around the topedge, the conductor and/or any plating on the conductor is not damagedduring the cutting /bending/bonding process.

A discussion of using the additional couductive layer(s) as a heat sink,spreading heat away from the die, is presented below. It should beappreciated that the pedestal 600 could be left in place to function asa heat sink, in which case it would need to be at least partiallynon-conductive (namely the top edges of the walls in contact with theleads) to prevent shorting the leads. Anodized aluminum is suitable.Also, a thermally and electrically conductive pedestal, having anelectrically non-conductive material (e.g., plastic) formed atop thesidewalls.

ADDITIONAL CONDUCTIVE LAYER(S) ACTING AS A HEAT SINK

As mentioned above, the additional conductive layer (e.g., 340, 460) canbe thicker and stiffer than a conventional additional foil layer (e.g.,220). Hence, the additional conductive layer can not only "rigidize" theTAB tape, but it can also act as a heat spreader, of sorts.

FIG. 7A shows a generalized view of a semiconductor device assembly 700having a top patterned layer 710, a plastic film layer 720, asemiconductor die 730 and an additional conductive layer 740 (similar to340 or 460), as set forth above, wherein the additional conductive layer740 is formed as a square ring. (The adhesive, e.g., 350 is omitted fromthis view.)

FIG. 7B shows a heat sink 770 formed integrally with the additionalconductive layer 740. In essence, the heat sink is simply a flat baseplate formed entirely across the bottom of the additional conductivelayer 740, more particularly spanning an area directly under the die730. A suitable thermally conductive paste 780, such as silver-epoxy, orthermal grease, is disposed between the top surface of the heat sink 770and the bottom surface of the die 730.

FIG. 7C simply shows that the flat base plate of the heat sink 772 canbe formed separately, rather than integrally (FIG. 7B) with theadditional conductive layer 740. A thermal adhesive or grease (notshown) is preferably disposed between the heat sink and the die, as inFIG. 7B. A suitable adhesive, such as epoxy, not shown, can be used tojoin the heat sink 772 to the additional conductive layer 740.

FIG. 7D shows a semiconductor device assembly with two additionalconductive planes 740 and 760 beneath an insulating layer 720 and asignal layer 710, and including a die 730, all similar to FIG. 4A. Aninsulating layer (e.g., 470) between the two additional conductivelayers 746 and 760 is omitted, for clarity.

In this case, the heat sink 790 is a separate base plate (similar, inthis regard, to the heat sink 772 of FIG. 7C). However, the heat sink790 is provided with a raised portion 792 sized and shaped to contactthe bottom of the die 730. Thermal adhesive or grease would be usedbetween the button 792 and the die 730. The heat sink 790 is furtherprovided with a plurality of fins 794 on a side opposite the button 792(away from the die 730) to aid in convective cooling of the die. Thevertical arrows "↑↑↑" indicate that the heat sink 790 is being broughtinto contact with the die 730 and lower underlying additional conductivelayer 760.

FIG. 7D also shows how the die 730 (e.g., 330, 430) is ultimatelyencapsulated with a glob-top epoxy 755, or the like.

What is claimed is:
 1. Method of fabricating semiconductor deviceassembly, comprising:(a) providing a substrate having a conductive layerof signal traces and a plastic support layer supporting the signaltraces; (b) locating a die in a central opening of the plastic supportlayer; (c) connecting the die to inner ends of the signal traces using aTAB process; (d) affixing a second conductive layer having a centralopening to the non-trace side of the plastic support layer, so that aninner edge portion of the second conductive layer is exposed in theopening of the plastic support layer; and (e) breaking inner endportions of selected traces, one end of the inner end portions connectedto the die, the other end being a free end; (f) bending the selectedtraces; (g) bonding the free ends of the inner end portions of theselected traces to the exposed inner edge portion of the secondconductive layer; (h) breaking outer end portions of the selectedtraces, one end of the outer end portions extending beyond the plasticsupport layer, the other end being a free end; (i) bending the free endsof the outer end portions of the selected traces; and (j) bonding thefree ends of the outer end portions of the selected traces to an exposedouter edge portion of the second conductive layer.
 2. The methodaccording to claim 1, further comprising the steps of:providingelongated slits through the plastic support layer near each outer edgeof the plastic support layer, each slit having an inner edge and anouter edge; providing an extended outer edge portion of the secondconductive layer into an area under the slits in the plastic supportlayer; and cutting the selected traces substantially at the inner edgesof the slits, bending the cut selected traces past the plastic supportlayer, and bonding the cut and bent selected traces to the outer edgeportion of the second conductive layer.
 3. The method according to claim1 wherein the second conductive layer is sufficiently thick to providesubstantial mechanical support for the plastic layer and the conductivelayer of signal traces.
 4. The method according to claim 3, furthercomprising the step of attaching the die to the second conductive layer,wherein the second conductive layer functions as a heat spreader byremoving heat from the semiconductor die.
 5. The method according toclaim 1, wherein the inner and outer end portions of the selected tracesare bonded by tape automated bonding (TAB).
 6. The method according toclaim 5, wherein the inner and outer end portions of the selected tracesare bonded by bumpless TAB bonding.
 7. The method according to claim 5,wherein the inner and outer end portions of the selected traces arebonded by thermosonically bumpless TAB bonding.
 8. The method accordingto claim 1 wherein the insulating layer is a first insulating layer, themethod further comprising the steps of:providing a second insulatinglayer having an inner peripheral edge defining a central opening, saidsecond insulating layer disposed on an opposite side of the secondconductive layer from the plastic support layer; providing a thirdconductive layer on a side of the second insulating layer opposite thesecond conductive layer and extending within the central opening of thesecond insulating layer, an inner edge portion of the third conductivelayer exposed within the opening in the second insulating layer; forminga second substrate by putting together the conductive layer of signaltraces, plastic support layer, second conductive layer, secondinsulating layer and third conductive layer; cutting selected othertraces substantially at the inner peripheral edge of the firstinsulating layer; bending the cut selected other traces past the plasticsupport layer, past the second conductive layer and past the secondinsulating layer; and bonding the cut and bent selected other traces tothe exposed edge portion of the third conductive layer.
 9. The methodaccording to claim 8 further comprising the steps of:providing firstelongated slits through the first insulating layer near each outer edgeof the first insulating layer, each slit having an inner edge and anouter edge; providing an extended outer edge portion of the secondconductive layer into an area under the first elongated slits in theplastic insulating layer; cutting the selected traces substantially atthe inner edges of the first elongated slits; bending the cut selectedtraces past the plastic insulating layer; bonding the cut and bentselected traces to the outer edge portion of the second conductivelayer; providing second elongated slits through the outer edge portionof the second conductive layer; providing third elongated slits throughthe second insulating layer and aligned with the second elongated lists;providing an extended outer edge portion of the third conductive layerinto an area under the third elongated slits in the second insulatinglayer; cutting the selected other traces substantially at the inneredges of the first elongated slits; bending the cut selected othertraces past the first insulating layer, through the second elongatedslits in the second conductive layer and through the third elongatedslits in the second insulating layer; and bonding the cut and bentselected other traces to the third conductive layer.